1. Field of the Invention
This invention relates generally to building construction methods and apparatus, and more specifically to an improved cable hold down and bracing system that ties building components together or to the ground, providing stabilization and resistance to uplift or overturning caused by wind, ground movement, earthquakes, or similar forces.
2. Description of the Prior Art
To reduce the effects of earthquakes, ground movements, hurricanes, typhoons, tornadoes and similar destructive forces, structures must be constructed to resist uplift or overturning forces. The most common structural uplift stabilization technique in use today is the nailing of plywood to structural framing members, creating a shear diaphragm. Shear diaphragms are then secured to adjacent diaphragms and to the foundation, forming a unified structure resistant to uplift and/or overturning.
The devices that are used to secure the shear diaphragms together and to the foundation are referred to as "hold down(s)". The most widely used "hold down" is in the form of a metal "L" shaped bracket, such as that described in U.S. Pat. No. 4,665,672. "Hold down" brackets are designed to be attached to a vertical framing member above the foundation, and to the foundation at each end of the shear diaphragm. This is typically done by: (a) through-bolting multiple L-shaped brackets to vertical framing members with two or more bolts; (b) interconnecting vertically-aligned L-shaped brackets using threaded rods; and (c) connecting L-shaped brackets to the foundation using threaded rods that have been imbedded within the foundation. This technique is repeated wherever vertical framing members are separated by horizontal (or perpendicular) framing members, and is also practiced in a horizontal manner to interconnect shear panels or remote building members.
There are significant disadvantages to the "hold down" technique described above:
1. Slack created in the threaded rods from shrinkage of the wood framing members is common, creating impact loads leading to a greater risk of failure; PA1 2. Many components of varying sizes are required, resulting in large inventory requirements; PA1 3. Precise alignment of anchor bolts and thoughholes for threaded rods and installation of same is difficult and very time-consuming; PA1 4. A high degree of labor skill is required, due to the complexity of the system and the precise alignment of holes/brackets; and PA1 5. Installation of the brackets must be staged during different phases of the construction process, resulting in increased cost and increased number of inspections by building officials. PA1 1. Lower costs of materials and assembly. Fewer components are required, resulting in smaller inventory requirements; less diversity in parts is required; no special (i.e., higher wage) labor skills are required; alignment of brackets and holes is more forgiving, reducing complexity and time of installation; and installation can be done all at once, rather than spread across several stages of construction. PA1 2. Improved structural integrity throughout a structure's life cycle. The inventive system eliminates impact loads which commonly occur from wood shrinkage; and maintains evenly-distributed compression throughout the structure. PA1 3. The inventive system is applicable to wider range of structures. It can be used to retrofit existing structures with little or no removal of wall coverings; and can be easily installed in mobile homes and modular building structures. PA1 4. The inventive system enables ease of inspection. It can be installed all at once (rather than in phases), reducing requirement for multiple inspections; and it can be inspected after the structure is completed, since primary components are located in the foundation (basement or crawl space) and attic (under the roof).
Furthermore, in extreme conditions such as during an earthquake, traditional hold down techniques may be inadequate. An earthquake releases moving point forces which are directed upward and sideways through the foundation to the framing members of a structure. These forces can cause a structure's shear diaphragms to rotate, buckle and/or separate, which can lead to the failure of the structure.
Rotational and point loading can be absorbed by the framing and shear diaphragms without over stressing the structure. This is attained by restricting the movement of the top corners of the shear diaphragms relative to the foundation, and by preventing separation of the framing members and shear diaphragms.